Currently each tag only has one question (and neither tag has a wiki summary).
For medical the question was What medical conditions can impede someone from going into space?.
medical would seem to be more concerned with medical treatment and (as in the sole existing question) interactions of medical conditions with space travel. (Medical research questions may or may not be on-topic.)
Someone with a clue should write wiki summaries that would help distinguish these two related tags. (I am assuming that they are not so closely related as to be synonyms.)